1891.] The Heliotropism of Hydra. 415 
bottom. It is, however, difficult to determine the precise character 
of the negative: heliotropism, since it only occurs at an intensity 
that is unfavorable to the general condition of the aquarium, and 
thus indirectly injures the Hydras. 
Up to this point there is no essential difference in the behavior 
of the two species, although, as many observers of Hydra have 
pointed out, the.movements of H. viridis are more rapid than 
those of Æ. fusca, so that the former species almost invariably 
leads the march towards the light. If now the aquarium be 
allowed to stand for a long time undisturbed (the water remaining 
unchanged, but maintained at a constant level), until the food 
supply of Daphnia, Cypris, etc., becomes scanty, a very interesting 
series of movements may be observed in H. fusca, (They are 
only occasionally performed by H. viridis, and never, so far as I 
have observed, with the same regularity as in the former species.) 
After a prolonged stay near the surface the animal detaches itself 
from the the glass, and with tentacles widely outstretched sinks 
slowly to the bottom, often floating for a time at the surface before 
the descent. Arrived at the bottom, it slowly crawls once more 
to the light side, gradually, and with many deviations from the 
straight course, reascends to the surface, ultimately sinks again 
to the bottom, and so on. Thus the movements pass through a 
cycle, extremely variable in its details, but on the whole maintain- 
ing the character of a slow and regular rotation. The duration 
of the cycle is extremely variable; it may be only one or two days, 
or it may be as many weeks.‘ 
What is the use of these movements, and by what stimuli are 
they called forth ? 
HI. Purpose and Cause of the Movements.—It appears to be 
commonly assumed that Hydra moves towards the source of 
light “ for the sake of warmth, ’—ż e., that within suitable limits 
a higher temperature is more agreeable to the animal or more 
* In order to realize the truth of this description it is necessary to have under obser- 
vation a large number of individuals in a large aquarium, to which they have become 
thoroughly accustomed by a residence of weeks or months. Many of my observations 
have been made on a fraternity of Hydras from five hundred d strong, @ 
ch had arisen in the aquarium from a group of three or four 
course of about two months. In this fraternity the cyclical character of the movements 
was very marked, and the descent of the animals might be observed al —— 
the 
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